“Sing out, Louise!” begins one of the most famous entrances in show business and one of the greatest musicals of all time. Mama Rose stops a rehearsal.

Even though lauded by critics and audiences alike, and with Ethel Merman knocking them pretty much dead as Mama Rose, during the original run, Gypsy had a problem.

Just before the final scene, Mama Rose stands alone on the stage. For years and years she encouraged — nay, drove her two daughters to excel. Both did: one became June Havoc, the other, Gypsy Rose Lee. But no thanks to Rose.

Rose is alone, even inside, where her mind’s going on tilt. So her imagination fills in the gap. She’s in the spotlight on a vaudeville stage and it’s her “turn” — finally! — to shine.

She sings “Rose’s Turn,” and like a starved fire seeking the whole world for fuel, she unleashes her frustrations and dented dreams.

The song’s a profound soliloquy matched in musical theater only by Billy Bigelow’s “Soliloquy” in Carousel.

Every time Merman belted it, audiences broke the applause meter. But they weren’t supposed to. Rose was spent, maybe half or three-fourths mad. The number had to flow directly into the final scene without a break. Merman can’t suddenly drop character and acknowledge a thousand “bravas” — as Merman, not long-suffering Rose.

“Rose’s Turn” became the show-stopper that shouldn’t be. Lyricist Stephen Sondheim urged composer Jule Styne not to break the illusion. “To have a mad scene and then have a bow,” he wrote, “violated everything that I thought I had learned from Oscar Hammerstein, who taught me to be true to the character and true to the situation.”

What to do?

The original solution didn’t please anyone: take all the air out of the balloon. Rose repeated the final words “for me,” over and over until the last dissonant chord, as if out of her mind. No applause at all. Then — whoosh — final scene. But that balloon was gigantic. Audiences wanted to praise, extol, and give thanks to what they had just witnessed — and who cares if it broke the fourth wall?

Even Oscar Hammerstein thought so. At Gypsy’s tryout in Philadelphia, he told Sondheim, “The audience is so anxious to applaud her, they are not listening to the scene that follows. Since the scene that follows is what the entire play is about, you must let them release themselves. That’s what the applause is for.”

Arthur Laurents, who wrote the original book, directed the 1973 revival, with Angela Lansbury as Rose. Laurents devised a brilliant solution for the “to “stop or not to stop” question.

Do both.

Lansbury sang “Rose’s Turn” (and earned a Tony Award for her performance). At song’s end, the audience clapped their hands raw, and Rose accepted the adulation. But it was Rose, not Lansbury. When the applause ceased, Rose still heard it, still bowed, as if the whole theater had just fulfilled her dream of a lifetime.